In general, a commercial pure titanium has usually been processed such that, using sponge titanium obtained by the Kroll method or titanium scrap as the melting material, the material is melted by vacuum arc remelting (VAR), electron beam remelting (EBR), or the like to be made into a large-sized cast product (an ingot). Here, as the shape of the cast product, only a circular columnar cast product (a billet) is possible in the case of vacuum arc remelting, whereas casting into a rectangular cast product, that is, a slab is possible in the case of electron beam remelting.
When using such a large-sized cast product as the material to produce a titanium material such as a titanium thin sheet, the large-sized cast product is subjected to surface cutting mending as necessary, is then subjected to slabbing or forging in a hot condition, and is thereby made into slabs with a shape and dimensions suitable for subsequent hot rolling. The hot working step by slabbing or forging is herein referred to as a breakdown step. Then, generally it has been the case that, after the surface is subjected to cutting mending of cutting approximately several millimeters in order to remove an oxide layer or an oxygen-concentrated layer formed on the surface of the slab after the breakdown, the resulting piece is subjected to hot rolling.
However, in such a conventional common method, a great deal of time and cost are required for the breakdown step by slabbing or forging for fashioning from a large-sized cast product to a shape and dimensions suitable for hot rolling, and this has been a severe bottleneck to the improvement in productivity of titanium thin sheet production and cost reduction.
As a method for casting a slab-shaped cast product, recently a direct cast (DC) slab casting method (direct casting method) has been employed in which, in place of large-sized ingot casting like that described above, titanium molten metal melted in a hearth by electron beam remelting is continuously poured into a water-cooled copper mold kept in a vacuum atmosphere, the portion solidified in the water-cooled copper mold is continuously drawn out from the lower end side of the mold, and thus a slab-shaped cast product with a prescribed length is obtained. By the DC slab casting method, a technology of producing a relatively thin slab-shaped cast product, that is, a titanium cast product having a shape and dimensions that allow the piece to be subjected to hot rolling as it is being established.
When such a method of electron beam remelting and DC slab casting in a vacuum is employed, a breakdown step that has been needed can be omitted, and as a result it becomes possible to improve the productivity of titanium thin sheet production and reduce production cost. However, also in a slab obtained by DC slab casting in a vacuum, the surface layer of the cast product as it is has severe concavities and convexities and a large number of defects. If such a cast product is subjected to hot rolling as it is, the surface properties of the sheet after hot rolling (the hot rolled sheet) are deteriorated; thus, the fact of the matter is that, as in the case where a breakdown step starting from a large-sized ingot is used as described above, the resulting piece can be subjected to hot rolling only after cutting is performed on the surface. Therefore, the yield of the material is reduced, and the time and effort and cost for cutting are required; hence, the fact of the matter is that the demand for further improvement has been strong.
Further, even when a slab obtained by employing the method of electron beam remelting and DC slab casting in a vacuum in the manner described above (a breakdown step being omitted) is subjected to hot rolling after surface cutting is performed on the slab, there is a problem that the surface properties of the hot rolled sheet after hot rolling are not necessarily satisfactory. That is, there is a problem that a large number of large and small overlying flaws with lengths of approximately several millimeters to 10 mm occur on the surface of the hot rolled sheet. Such a large number of overlying flaws of the surface are herein referred to as surface defects. It is presumed that such surface defects of the hot rolled sheet are derived from a coarse cast structure of the cast slab. That is, it is presumed that the slab that has not undergone a breakdown step, which is hot working, has a cast structure formed of coarse crystal grains that is as cast; and even when cutting is performed on the surface, a coarse structure exists in the surface layer after cutting, and surface defects occur on the hot rolled sheet due to such a coarse surface cast structure.
Here, as a specific factor by which surface defects occur on the hot rolled sheet due to a coarse cast structure, it is presumed that concavities and convexities are formed on the surface by the influence of deformation anisotropy in grains and between crystal grains due to coarse crystal grains and, with the progress of subsequent hot rolling, metal lies over the concavities and becomes surface defects. Further, in a titanium alloy, the α phase (the grain boundary a phase) is created near the grain boundaries between prior β crystal grains during transformation. In an alloy system containing a large amount of an α-stabilizing element(s) such as Al or O, which is commonly used in titanium alloys, the hot deformation resistances of the α phase and the β phase are greatly different, and the difference may form a starting point of a crack during hot and cold working to be performed later.
For a titanium slab for hot rolling obtained without undergoing a breakdown step, several methods of performing modification treatment on the surface layer of the slab before hot rolling have already been proposed in order to prevent the occurrence of surface defects on the surface of the hot rolled sheet after hot rolling.
For example, Patent Literature 1 proposes a method in which the surface of a titanium slab for hot rolling is beaten with a steel tool having a tip shape with a curvature radius of 3 to 30 mm or a steel ball with a radius of 3 to 30 mm in a cold condition (subjected to plastic processing), and is thereby provided with dimples with an average height of the profile elements of undulation of 0.2 to 1.5 mm and an average length of the profile elements of undulation of 3 to 15 mm. In the proposed method, by providing the surface layer of the titanium slab with a prescribed plastic strain in a cold condition with a steel tool or a steel ball like that described above, the surface layer is recrystallized during subsequent hot rolling, and thus a fine structure is produced. Thereby, the occurrence of concavities due to a coarse structure like that described above can be prevented, and therefore the amount of surface defects of the hot rolled plate can be reduced even when a breakdown step is omitted.
Patent Literature 2 proposes a method in which high energy is applied to a surface of a titanium slab for hot rolling, particularly to a surface on the side serving as a rolling surface during hot rolling, by high-frequency induction heating, arc heating, plasma heating, electron beam heating, laser heating, and the like and thereby only the surface layer is melted to a depth of more than or equal to 1 mm, and immediately thereafter rapid cooling re-solidification is performed. In the case of performing the proposed method, since the melting point of titanium is naturally a temperature higher than or equal to the β transformation temperature, in association with the melting of the surface, also a heat-affected zone (HAZ) layer on the lower side (matrix side) of the molten layer of the surface is heated to higher than or equal to the β transformation temperature, and transform into β phase. In the proposed method, the surface is smoothed by the melting of the surface layer of the titanium slab for hot rolling, then the molten layer is rapidly cooled and solidified by heat removal from the matrix side, and at the same time the HAZ layer (the β phase) on the lower side is rapidly cooled; consequently, the molten layer and the HAZ layer become a fine transformed structure (usually, a fine acicular structure). The surface layer that has been made fine in this way is recrystallized in the early stage of subsequent hot rolling, and becomes a fine grainy structure with random orientations (an equiaxed grain structure). Therefore, it has been possible to some extent to prevent the occurrence of concavities due to a coarse structure and also eliminate the surface defects of the hot rolled sheet after hot rolling. However, in the invention disclosed in Patent Literature 2, there is a case where the surface defects of the hot rolled sheet cannot be prevented at a practical level, and the cause thereof is unclear; thus, the improvement therein has been desired.